RAM SSDs versus flash SSDs?

(updated
with a new introduction which describes the historic market perspective
followed by the original article text )

..

Context and a historical
perspective

The original popular classic article below
(which included contributions from 10 of the leading SSD companies at the
time) was published 10 years ago in
August 2007 at
a time when the enterprise rackmount SSD market was still dominated by
RAM SSDs and there were
many questions about how the market was going to change.

To put this
article into context - with the benefits of 20 20 hindsight.

It
was the same month that the 1st enterprise rackmount array of flash SSDs
appeared from EasyCo
with a flash management scheme which reduced
write amplification
in notebook
SSDs and boosted throughput. In those days - you could burn out most flash
SSDs in a few months if you placed them into a high
IOPs system.
Converting unreliable
flash memory into
reliable SSDs
remains a design challenge today - because
flash is getting
intrinsically worse as geometries shrink - while markets demand increasing
SSD performance.

It was the month before
Fusion-io's first
PCIe-SSD shipped. (It was great on performance - but still didn't include all of
the fault
tolerance features which were introduced in later models.)

By
2012 - RAM
SSDs had become a niche in the enterprise SSD market accounting for less than
one per cent of systems. And for several years after that the role of DRAM in
what became known as "all flash arrays" was simply as a
cache to the flash.

You might have expected that the story would end there.
Flash had won the storage
war. But 2015
we saw a new phase of this flash-v-DRAM memory war - with the heating up of the
SCM SSD DIMM
wars market. It then looked as though flash memory might challenge DRAM in
the memory heartland of core memory (and not just as the preferred memory type
in solid state storage). At the heart of this new battle was another new
value
proposition that large capacity and low latency flash (or other alt nvm)
could be indistinguishable from DRAM in applications efficacy at much lower
system cost.

This tiered memory concept (when it becomes supported and
proven in enough software environments) could reduce the size of the DRAM
market in the same way that SSDs reduced the size of the
hard drive market.

The "rethinking
RAM" market in 2017 is in a similar state of explosive opportunity
that the "rethinking RAM-SSD" market was back in 2006.

But
let's get back to the original text of the story which appeared at this url:-
RAM SSDs versus Flash SSDs - which is Best? - War for the datacenter core.

There is no single "right
answer" to the question - which technology is best. It depends on the
business application and the performance / cost trade offs of reaching
higher server performance.

Revenues for companies in both SSD market
segments (RAM and flash) have been growing fast in recent years. That may
change for individual companies due to lower average selling prices and
competitive alternatives. Lower ASPs will come from falling memory prices
(over the long term) and shorter term from aggressive competition from new
SSD market entrants who want to buy market share. Despite the inevitable
shakeout of many SSD oems - I expect the
number of SSD
vendors to continue growing for the forseeable futures and overall revenue
for both SSD technology segments to continue high double digit growth.

Memory
Channel SSDs - these are low latency flash SSDs in a DRAM form factor -
which transfer data via the DRAM channel

hybrid DIMMs
- these are RAM modules - which back up their temporary data to and on board
flash SSD when the power fails

.

In August 2007
Violin Memory launched
the world's fastest 2U SSD and the densest SSD using RAM..

It was the
first oem to publicly announce that it is developing SSD products with both
RAM and
flash technologies.

DRAM vs. Flash SSDs

A
Perspective from Morgan Littlewood, VP Marketing and Business Development at
Violin Memory

It seems unlikely that a single technology will ever
excel across all key dimensions of a storage system, including performance,
capacity and price. The obvious analogy is that while the world's goods are
transported via planes, trucks, trains and ships, each of these markets
continues to grow as trade increases. Performance, capacity and price are also
important attributes of a transport system.

Each core technology has its own limitations. Extracting more IOPS
from disks is increasingly difficult and, hence, disk developments focus on
capacity and bandwidth enhancements. Flash developments focus on capacity and
cost effectiveness, but as a result, performance and reliability are being
negatively impacted. DRAM developments focus on cost and performance
improvements, but are constrained by reliability requirements.

The table below of storage technologies illustrates the unique
attributes of each technology.

Capacity hard disks have a cost/GB
which is significantly better than solid state.

Flash SSDs are very
well suited to read-only applications (and mobility applications).

DRAM
SSDs have costs per IOPS which are much better than Performance hard disks or
Flash SSDs.

The success of solid state will undoubtedly have most
impact on the performance HDD market.

Table of Storage Technologies and Key
Metrics - source Violin Memory

Technologies

Capacity (GB)

Latency (µS)

IOPs

Cost / IOPs ($)

Cost / GB ($)

Capacity HDDs

2,500

12,000

600

13.3

3

Performance HDDs

700

7000

1,200

16.6

28

Flash SSDs

700

200

500

140

100

Flash SSDs (read only)

700

45

50,000

1.4

100

DRAM SSDs

250

3

200,000

0.5

400

Notes

1 -
Metrics are all normalized as typical rackmount system statistics per U
of height (1.75")

Within a single Enterprise,
there may be thousands of applications and each of these has its own
performance, cost and capacity issues. Even a common software application (e.g.,
database) can have a wide variety of requirements depending on the size of the
databases, their update rates and the rates and complexity of queries the users
require. Solid state solutions must be able to scale to cover this wide range of
requirements.

The scalability (capacity, density and performance) of
solid state solutions will determine the size of the market opportunity.
Improved density has the secondary effect of driving down both capital costs and
the operating costs associated with power and cooling. Scalable memory
solutions increase the affordability of the technology to a wider range of
applications and will drive market growth.

DRAM and Flash technologies will co-exist with disk systems and
address the performance and reliability constraints of rotational media. The
unanswered question is: "How will each of these technologies best be used?"
Is Disk emulation the only solution? Why connect to DRAM via a slow disk
attachment interface like Fiber Channel? DRAM has read and write latencies so
low that it can be used as an extension of server memory. Flash reduces the need
for duplicate data copies by dramatically accelerating read-only storage. It is
not clear whether emulating the 4K blocks of hard disks is the most efficient
long term use of solid state technologies.

Violin has designed memory appliances to maximize the technical
benefits of DRAM and Flash technologies either separately or in combination.
DRAM and Flash are installed as high-density plug-in modules. Both disk
emulation and other APIs are supported as configuration options. The objective
is to enable the enterprise with optimal solutions that meet their specific
needs, for a broad range of applications.

In summary, "Solid State Drives" is too limiting a term for
the market segment. Emulating a spinning disk is a not a great aspiration for
solid state technologies, which are radically higher performing. Then again, car
engines are still measured in horse power!

It discusses considerations in choosing the right
technology and provides a viewpoint on performance comparison using
industry-standard benchmarks and handy graphical guides for estimating read and
write database performance improvements.

Solid Access
Technologies shipped the industry's first
serial SCSI
compatible SSD in April 2005. They also make parallel SCSI and FC models. That
gives them a unique insight into a wide range of enterprise customer speedup
applications.

Regardless of which type is purchased, every SSD sale helps to
accelerate the SSD solution concept into the mainstream user's mind-set. Flash
SSDs have received a lot of media buzz, and for some customers it is the perfect
solution. However, they simply won't cut it in demanding enterprise environments
because of their limitations relative to write cycles durability, write
performance, connectivity and device leveraging (multipathing), not to mention
multi-bit error correction capability. Fortunately, SSD customers have plenty of
options.

As the producer of the ultra-fast RAM SSD Solid Access USSD 200, we
know that our USSD 200 solution is, for many applications, the most effective
solution. It can process, via just one single FC port, 95,000 IOPS (read
or write, random or sequential) and 55,000 IOPS via a single wide SAS port. For
more than 2 years, it has been the only
SAS-interface SSD
available.

All of our customers could have deployed flash SSDs. Why didn't
they? Because a thorough evaluation of their application issues and
potential solutions led them to USSD 200. They did what every customer should do
they paired a specific problem with the right solution.

SSD OEMs naturally showcase the highest IOPS numbers reached with the
shortest data blocks. However, because applications utilizing SSD typically use
4Kb, 8Kb , 16Kb block sizes, the combination of IOPS and data throughput is a
more relevant measure of the overall application performance boost. For example,
absolute IOPS numbers for USSD 200 FC are higher than for SAS, but the USSD 200
SAS offers twice as much data throughput for read or write (almost 800MB/s
sustained via each wide port) than Fibre Channel, making SAS the fastest SSD
connectivity out there. SAS is also the fastest growing; by the end of 2008,
Gartner predicts data storage interface representation of 41.2% SAS, 22.9% FC,
and 7.3% SCSI. On top of that, SAS HBAs cost 4x less than
FC HBAs.

Assertions of superiority of one technology over another actually
miss the point. The real issue here is transparency. Performance and pricing
data should be clearly and accurately presented to the consumer; then, let the
customer decide which technology best suits their needs. At Solid Access, we
publish both our price list and our benchmarks. Furthermore, if a potential
customer sends us an IOmeter script based on their app patterns, we will run it
on USSD 200 FC, SAS or SCSI and generate a test result report for them.
Potential customers may also test the SSD solution before committing to buy.

In short, we believe that an unambiguous presentation of pricing and
performance data places the customer in the best position to make an informed,
sound decision regarding their SSD needs.

With all the buzz that is surrounding flash-based
enterprise SSDs these days it would be natural to assume that the DRAM SSD's
days are numbered.

Quite the opposite is the case.

Enterprise flash SSDs offer very fast read speeds, but since they are based on
NAND flash they cannot match the speed of a DRAM device. DRAMs have truly
random access, where NAND flash has a relatively long latency before bursting
out data. Although NAND's 25 microsecond latency is 80x as fast as the 2
millisecond average latency of a 15K HDD, it still is over 1,000x slower than
the 20 nanosecond maximum access of a typical DRAM.

DRAMs also have
significantly faster write speeds than those of NAND, and are byte-writeable:
Any byte of the DRAM can be written to at any time at about the same speed as
its read speed. A NAND chip requires an entire 2K-byte page to be written at
any one time, and before that happens, a 256K-byte block must first be erased,
taking up to 10 milliseconds. To this end enterprise-class flash SSDs use a
small DRAM array to accelerate writes.

A flash-based enterprise SSD is really good at accelerating an
enterprise system at a modest price ($5-10,000) with few changes to the system's
software or hardware. The degree of acceleration depends upon the system's
configuration before the SSD was added. Systems that taxed the ingenuity of the
IT team - ones that use RAID or short stroking to coax more bandwidth out of
enterprise-class HDDs - will see the greatest benefit. The less such
techniques are used, the less the flash SSDs will be able to help.

DRAM SSDs have always been for the most extreme cases. If a flash SSD can be
likened to using dynamite, then a DRAM SSD can be compared to a nuclear bomb.
The system is physically larger than a flash SSD, using a rack or cabinet rather
than an HDD's form factor, and the price may be a cool half million or so, but
the cost/benefit tradeoff can be compelling for those few who need everything
and more.

While a flash SSD can reduce the number of enterprise HDDs a system
requires, DRAM SSD makers tell us that the addition of one of their systems can
eliminate the need for a ten or more entire servers.

So, are DRAM
SSDs threatened by flash SSDs? Not at all! As a matter of fact DRAM
SSD makers have told us that the heightened awareness of SSDs in general has
brought prospective customers to them who may otherwise have remained aware that
these extraordinary devices exist.

Memoright makes the
industry's fastest shipping 2.5" SSDs (October 2007). They are targeting
the hard disk replacement market - but their products can also be useful in the
server acceleration market.

Introducing the Fastest 2.5"
Flash SSD

by Jasmine Hong, Sales Manager, Memoright Memoritech

With pricing trends stable /increasing in Nand Flash post the release
of the Apple Iphone, it appears that the concerns over SSD encroachment have
eased and we could see significant SSD penetration in the notebook market over
the next few years. More and more manufacturers have already begun to introduce
SSDs with varying densities and transfer interfaces, while many kinds of Brand
SSD have entered the market in succession. However, there is a big difference
even among similar looking SSDs in the market.

SSD's storage media is generally based on RAM and Flash. Historically
RAM based SSDs were faster than flash based products - but a RAM based SSD is
much expensive than Flash based SSD and is not usable in all the same
applications for example notebooks.

(Many
flash SSD oems use licensed host interface cores which were originally
designed for slower hard disk drives - Editor.)

Memoright's patented
technology (which also includes balanced multi-channel read/write) enables
their SSD to deliver 100M Bytes/sec sustained read and write transfer rates -
which is the fastest throughput in a currently available 2.5" flash SSD,
while its random access performance is typically 5x faster than a hard disk.

Among the inherent advantages of Flash SSDs over DRAM are
lower power consumption, higher density in smaller footprints, ruggedness of
solution, security (securErase), non-volatility and price.

As far as applications are concerned, each application would have
different requirements in terms of performance, ruggedness, security, and
footprint. Flash SSDs would inherently be strong in military/industrial apps and
DRAM SSDs may have an edge in enterprise applications due to some performance
advantages. However, DRAM SSDs are really expensive compared to flash SSDs,
which offer significant performance improvements (vs HDDS) that meet the
requirements of business enterprises.

The random I/O performance of DRAM is superior but the device is more
expensive. Flash is way cheaper than DRAM and can also provide better random
performance than HDDs. Price of flash memory is also going down, making flash
SSDs even more palatable to the enterprise market.

Applications such
as OLTP, databases, would require high random performance and these are the
types of applications that DRAM or flash would be very strong in. However, flash
SSDs gets the plus points with regard to non-volatility (they don't require
batteries for backup), capacity (denser memory chips due to R&D advances)
and form factor (drop-in replacements for HDD).

I asked
Attorn for a
contribution to this article because its HyperDrive4 (available in 5.25"
CD drive form factor) stands at the threshold of entry level RAM SSDs.

In
August 2007 the company launched a rackmount HyperDrive4, which the company said
(at $250 per gigabyte) was the lowest price per gigabyte for a RAM based solid
state drive.

"RAM versus Flash SSDs
- which is Best?"

by Jan Wels, Attorn

Our RAM SSD
products based on the HyperDrive4 are able to solve various application issues
that were not previously being solved by alternative methods of archival and
retrieval. This is due to the constant available high IOPS and STR regardless of
the setting in which it is applied, be it in web or file servers, databases or
as replacement of in-server memory systems.

There is no other
technology that can match the performance of a RAM SSD, especially in an
environment where there are a significant number of write operations.

Of course for many applications the performance will not be so
critical that the investment cost of a RAM SSD is warranted and a Flash SSD will
offer a good alternative.

For the foreseeable future we see a market where there is room for
both solutions alongside the normal hard disk. With HyperDrive4-array sales and
placements in multi billion dollar corporations around the globe our RAM SSD is
gaining market respect with each installation. The fact that this new product
leads the market from a pricing perspective has not disappointed customers from
a quality, reliability and most importantly performance perspective.

The
expectations for the fourth quarter of 2007 look good and we expect the
HyperDrive4 to gain further momentum in 2008. Development in the next 2 to 3
years of our RAM SSD's will result in a further increase of the performance and
we expect to reach 1,000,000 IOPS per drive and a STR of 300MB/s.

For the longer term we think that two factors will determine whether or not
there is room in the market for both RAM and Flash SSD's. First if the Flash
SSD's will be able to bridge the performance gap with RAM SSD's, and secondly
the development of the price difference between RAM and Flash memory.

In August 2007
EasyCo launched its
revolutionary "Managed Flash Technology."

Building arrays
from commodity
flash SSDs (but
addressing them in a completely new way) the technology results in system
write performance that is typically 100x faster than the bare solid state
flash drive.

MFT, which is shipping today in Linux servers, changes
the boundaries of flash SSD system performance.

Editor's
intro:- Douglas Dumitru has written a
white
paper which will become the classic reference for systems engineers who want
to understand the dynamics of flash versus hard disk performance running real
applications.

It begins by comparing in detail the performance achieved
by commodity flash SSDs for various read / write size blocks to that of high
performance hard drives. The dropoff in overall performance for various ratios
of reads to writes is also documented.

This article also briefly
notes and analyzes the performance of high end RAM SSDs and the fastest
commercially available flash SSDs.

Understanding these real world
constraints, the article leads on to an explanation of EasyCo's patent pending
Managed Flash Technology which delivers random
write performance in a flash SSD array which is almost 500x
the performance of the bare drives running RAID-5. This changes the mapping
scheme in the flash memory dynamically to work around contention caused by block
erases. This revolutionary approach works with commercially available flash
SSDs and is scalable. ...read the
article (pdf)

Conspicuous by its absence from
the first published version of this article, the reason for the company's
reticence on the subject of flash versus RAM SSDs became clear in September 2007
- when Texas Memory
Systems launched the RamSan-500.

At first glance it looks as if the
RamSan-500 takes a similar approach to flash write management as EasyCo's MFT
(see above) but TMS uses different design decisions - and implements the
process transparently within the hardware of the SSD instead of needing an OS
dependent driver.

Flash
memory is poised to become a major player in datacenter storage systems. It
provides an all electronic method of storing data in silicon, removing the only
component in modern data processing systems that still relies on moving parts in
the data path: the spinning hard disk. Though Flash memory offers some
compelling advantages over disk-based storage, it is not well understood in the
storage industry. This
paper
describes some the properties of Flash memory and then explains how the
RamSan-500, produced by Texas Memory Systems, leverages its strengths and
compensates for its weaknesses to offer the first enterprise ready Flash system.

Article
extract - Write Performance

"The RamSan-500, due to its design, achieves a much higher level
of Flash write performance, making the technology enterprise-ready. The large
battery backed DDR RAM cache is used to buffer write operations (random or
sequential) so host processing can continue after only the cache write. The back
end Flash controller takes the buffered writes and maps them to a new physical
location every time they are written, ensuring that new writes are always
written sequentially to locations in the Flash and thus avoiding the random
write Flash penalty."

The next time in SSD history
when there was so much signal of future change was probably
2014
when (among the many other big ideas taking hold of the imagination of the
market) we got the messages that "random access memory doesn't have to be
RAM" and confirmation that "in-situ SSD processing" would become
a future market force.

What we've got now is a new
SSD market melting pot in which all performance related storage is made from
memories and the dividing line between storage and memory is also much more
fluid than it has been before.

Increasing user
sophistication due to their experience of enterprise flash is changing the
way they view products in the market. And a new generation of enterprise
software is helping them to plan their solutions in ways which are abstracted
away from proprietary bundled features.

Prior to 2012 - knowing
the flash memory type in an SSD was very useful in anticipating the SSD's
behavior.

However, advances in SSD controllers and SSD architecture now mean
that the performance and reliability of some memory types can be stretched - for
example multiplying endurance by upto 100x, or multiplying speed - even when
using identical memory with different flash management schemes.

Consequently any foreknowledge which you may have about the internal
memory type coupled with the SSD specification tells you more about the costs
for the SSD vendor than it tells you about the SSD's likely characteristics.

Editor:- in December 2010 - I interviewed Jamon Bowen,
Director of Sales Engineering for
Texas Memory Systems
and asked him about the use of SSDs in financial applications like banks and
traders - a market which he said accounts for most of their RAM SSD sales.

The company which started in RAM SSDs
over 30 years
ago - now sells more flash SSDs than RAM SSDs (even though the product
brand for both types of SSD is
confusingly called
RamSan.) Bowen said that flash is 70% of their business.

Jamon
Bowen said that in many bank applications RAM SSDs are actually cheaper than
flash - because of the small size of the data. TMS still sell a lot of 16GB RAM
SSDs.

Production bank systems are typically shared by many hosts and
get a lot of write IOPS. To achieve the same reliability and latency with
flash would require over provisioning which would drive the cost up.

He
suggested a simple rule of thumb for intensive IOPS bank SSDs on the SAN

< 128GB capacity - RAM SSDs cheaper

128GB to 4TB capacity - middle ground could be either - or determined by
other constraints

> 4TB - flash SSDs cheaper

Jamon Bowen said that the analysis
side of operations in banks is different. That tends to have much larger data
sets and is more read than write intensive. In these apps - flash SSDs are
usually more economic.